A carbohydrate-binding module (CBM) is defined as a contiguous amino acid sequence within a carbohydrate-active enzyme with a discrete fold having carbohydrate-binding activity. The requirement of CBM's existing as modules within larger enzymes sets this class of carbohydrate-binding protein apart from other non-catalytic sugar binding proteins such as lectins and sugar transport proteins.
CBM's were previously classified as cellulose-binding domains (CBD's) based on the initial discovery of several modules that bound cellulose (Tomme et al. (1989) FEBS Lett. 243, 239-243; Gilkes et al. (1988) J. Biol. Chem. 263, 10401-10407). However, additional modules in carbohydrate-active enzymes are continually being found that bind carbohydrates other than cellulose yet otherwise meet the CBM criteria.
Previous classification of cellulose-binding domains was based on amino acid similarity. Groupings of CBD's were called “Types” and numbered with roman numerals (e.g. Type I or Type II CBD's). In keeping with the glycoside hydrolase classification, these groupings are now called families and numbered with Arabic numerals. Families 1 to 13 are the same as Types I to XIII (Tomme et al. (1995) in Enzymatic Degradation of Insoluble Polysaccharides (Saddler & Penner eds.) 142-163, American Chemical Society, Washington).
Presently the known CBM's a reclassified in families 1-6 and 8-33. Most classified CBM's are of bacterial origin, and the known fungal carbohydrate-binding modules are mainly classified in the family CBM1. However, representatives of fungal CBM's are also found in CBM13, CBM18, CBM19, CBM20 and CBM24. Until now, only the fungal carbohydrate-binding modules from CBM1 were known to bind to crystalline cellulose. The fungal CBM's from families CBM13, CBM18, CBM19, CBM20 and CBM24 have been shown to bind to substrates such as chitin, starch and mutan. However, also the fungal carbohydrate-binding modules from CBM1 bind very well to chitin.
A number of fungal cellulases has been shown to contain a CBD of family CBM1 consisting of 36 amino acid residues. Examples of enzymes known to contain such a domain are:                Endoglucanase I (gene egl1) from Trichoderma reesei.         Endoglucanase II (gene egl2) from Trichoderma reesei.         Endoglucanase V (gene egl5) from Trichoderma reesei.         Exocellobiohydrolase I (gene CBHI) from Humicola grisea, Neurospora crassa, Phanerochaete chrysosporium, Trichoderma reesei, and Trichoderma viride.         Exocellobiohydrolase II (gene CBHII) from Trichoderma reesei.         Exocellobiohydrolase 3 (gene cel3) from Agaricus bisporus.         Endoglucanases B, C2, F and K from Fusarium oxysporum.         
The CBD domain is found either at the N-terminal (Cbh-II or egl2) or at the C-terminal extremity (Cbh-I, egl1 or egl5) of these enzymes. There are four conserved cysteine residues in this type of CBD domain, all of which are involved in disulfide bonds. (Prosite, Swiss Institute of Bioinformatics).
A DNA sequence encoding a CBD from a given organism can be obtained conventionally by using PCR techniques, and, also based on current knowledge; it is possible to find homologous sequences from other organisms.
It is contemplated that new CBD's can be found by cloning cellulases, xylanases or other plant cell wall degrading enzymes and measure the binding to e.g. cellulose. If the enzyme activity is bound to Avicel® under the standard conditions described below, it can be assumed that part of the gene codes for a binding domain.
Examples of CBM-like polypeptides obtainable from plants are expansins. Expansins are not CBM's per se because they are not found encoded in the same amino acid sequence with an enzyme activity. However, it has been observed that isolated CBM domains can have expansin like activity on cellulose (Levy and Shoseyov, 2002 supra). Din et al. (Bio/Technology 9 (1991) 1096-1099) has reported that the CBD CenA from Cellulomonas fimi endoglucanase A is capable of nonhydrolytic disruption activity of cellulose fibers resulting in small particle release. Furthermore, it was shown that CBD CenA could prevent the flocculation of microcrystalline bacterial cellulose (Gilkes et al. (1993) Int. J. Biol. Macromol. 15:347-351). Similar phenomena were observed for other CBD's (Krull et al. (1988) Biotechnol. Bioeng. 31:321-327; Banka et al. (1998) World J. Microbiol. Biotechnol. 14:551-558; Gao et al. (2001) Acta Biochim. Biophys. Sin. 33:13-18).
CBM's are known to be used in applications as diverse as washing, treatment of textile, removal of dental plaque, purification of polypeptides, immobilisation of active enzymes, modification of cellulosic material, baking, manufacturing of biofuel, modification of plant cell walls.